This invention relates to an apparatus, system, and method for improving the dynamic range of a receiver. More particularly, this invention relates to an apparatus, system, and method for improving the dynamic range of a receiver in the presence of a narrowband interfering signal.
Duplex radio systems, such as a Code Division Multiple Access (CDMA) duplex radio system, use spread spectrum full duplex transmission to transmit and receive cellular telephone calls. In a CDMA radio system according to the IS-95 standard, information to be transmitted is typically digitized, encoded, and spread over a 1.23 Mega Hertz (MHz) bandwidth. A typical IS-95 subscriber unit transmitter uses Offset Quadrature Phase Shift Keying (OQPSK) modulation which contains both time varying phase and amplitude components. Thus, the transmitter output has an amplitude modulation (AM) component and is not a constant envelope signal. A CDMA base station uses Quadrature Phase Shift Keying (QPSK) modulation with both time-varying phase and amplitude components. At the receiving end, the wideband spread signal is despread, decoded, and converted back into analog form. Frequency selective filters, referred to as duplexors, are used in the subscriber unit to combine the transmit and receive signals together to permit simultaneous transmit and receiver operation.
FIG. 1 illustrates a conventional subscriber unit duplex system. The system includes a transmit (TX) path and a receive (RX) path. The TX path includes a Driver 110 for amplifying an OQPSK modulated signal, a TX Filter 120 for filtering the modulated signal, and a Power Amplifier (PA) 130 for amplifying the filtered signal for transmission. The signal is filtered by a Duplexor 140 and transmitted to a base station via an Antenna 145. A signal is received from the base station via the Antenna 145 and filtered by the Duplexor 140. Because the Duplexor isolation is not infinite, a portion, PTX-RX, of the transmit signal appears at the receiver input when the transmitter is turned on. The received signal is amplified by an RX Low Noise Amplifier (LNA) 150, filtered by an RX Filter 160, and mixed with a local oscillator (LO) frequency in a Mixer 170. The received signal is then fed, for example, to an IF filter.
As CDMA systems are deployed into systems using the current Advanced Mobile Phone System (AMPS) cellular telephone band, a potential source of interference exists between the wideband spread spectrum signal transmitted by an IS-95 subscriber unit and the relatively narrowband AMPS signal coexisting in the same frequency band. If an AMPS signal, or any narrowband signal, is present within the RF passband of the receiver when the transmitter is on, the AM component of the transmit signal is transferred onto the narrowband signal, due to third order nonlinearity in the LNA 150. This effect, which is referred to as crossmodulation, is well known in amplitude modulation systems.
Because the bandwidth of a CDMA transmit signal is relatively large (1.23 MHz), it is possible for this wideband TX crossmodulation signal to appear in the IF passband of the desired receive signal, depending on where the narrowband interfering signal is located with respect to the desired RX channel. In fact, for any narrowband interfering signal having a center frequency closer to the RX center frequency than the TX spread bandwidth plus half the RX channel bandwidth, the TX crossmodulation product falls in the IF bandwidth. In an AMPS/CDMA system, this means that any narrowband interfering signal having a center frequency within approximately 1.85 MHz of the RX center frequency creates undesired TX modulation within the RX IF passband. The net effect of this crossmodulation product is to mask the desired receive signal, thus reducing the sensitivity of the receiver. This effect is illustrated in FIGS. 2A and 2B.
FIG. 2A illustrates a TX modulated signal. FIG. 2B illustrates the TX modulated signal transferred onto a narrowband interfering signal. As can be seen from the shading in FIG. 2B, the TX crossmodulation falls within the desired RX bandwidth, masking the desired receive signal and reducing the dynamic range of the receiver by effectively increasing the noise floor of the receiver.
This effect can also occur in the Mixer 170 illustrated in FIG. 1. However, the attenuation of the RX Filter 160 and the Mixer 170's third order input intercept point can be selected to suppress the TX signal enough so that the Mixer 170 does not contribute any appreciable additional crossmodulation product.
The level of TX modulation appearing about the narrowband interfering signal varies linearly with the level of the narrowband interfering signal and the modulation index of the TX modulation (% modulation), the square of the amplitude of the TX signal appearing at the LNA input (PTX-RX in FIG. 1), and inversely as the square of the third order input intercept point of the LNA 150. Thus, the factors that contribute to the crossmodulation product are the amplitudes of the TX signal and the narrowband interfering signal, the modulation index of the TX signal, and the third order input intercept point of the LNA 150.
Of the factors contributing to the generation of the undesired crossmodulation products, the modulation index is fixed by the system and cannot be changed. The amplitude of the TX signal appearing at the LNA input can be reduced by specifying more TX-RX attenuation in the Duplexor 140. However, in a small subscriber unit, the practical limit of isolation that can be achieved is not sufficient to eliminate the generation of the crossmodulation product. It is also not practical to reduce the amplitude of the narrowband interfering signal appearing in the RX band, since any attempt to do so would also reduce the level of the desired RX signal.
Conventionally, the third order input intercept point of the LNA 150 is varied to minimize the generation of crossmodulation products. An example of an LNA with a variable third order input intercept point is the RF 9986 integrated circuit produced by RF Micro-Devices. Conventionally, the third order input intercept point of the LNA is set at a maximum level while the transmitter operates at relatively high power and then switched to a minimum level while the transmitter operates at reduced power. This improves the dynamic range of the receiver to some extent and also saves current, since the lower third order input intercept point generally requires less current from the power supply. In addition to controlling the third order input intercept point of the LNA, the gain of the LNA is typically also reduced under strong on-channel signal conditions to help improve receiver dynamic range.
While the conventional approach is effective for many situations, it does not provide adequate dynamic range of the receiver in at least one situation, e.g., when the subscriber unit is operating relatively close to a base station and a narrowband signal appears. In this situation, the subscriber unit transmitter is operating at reduced power, and the LNA's third order input intercept point, which is adjusted depending on the transmit power level, is set at the minimum level. If a strong AMPS signal or other narrowband signal, originating from an AMPS base station co-located with the CDMA base station, an AMPS microcell within the larger CDMA cell, or another source, appears within the +/−1.85 MHz region around the subscriber unit's RX channel, the strong narrowband interfering signal mixes with the subscriber unit TX signal and generates on-channel crossmodulation products. This causes a reduction in RX signal-to-noise ratio and possibly a loss of communications.
Linking the control of the LNA's third order input intercept point to the transmit power level is thus, not alone, sufficient to achieve the optimum dynamic range to deal with narrowband interfering signals that are likely to be encountered in actual system operation.
There is thus a need for method and system for increasing the dynamic range of a receiver in the presence of a narrowband interfering signal which overcomes the drawbacks of the prior art.